128 RESEARCHES ON FUNGI 



other of them ; but whether or not this supposition is a good one 

 can only be decided by further investigation. ^ 



According to Haberlandt,^ as we have seen, the reaction of the 

 leaves of the higher plants to heliotropic stimuli can be explained by 

 assuming that the epidermal cells of the lamina act as ocelli and that 

 heliotropic equilibrium is only attained when the spot of light formed 

 in each cell has come to lie symmetrically on the protoplasm 

 covering the cell's inner wall. If this theory is true, a lamina when 

 subjected to two equal beams of white light making a wide angle 

 with one another, say of 40°, should, like the sporangiophores of 

 Pilobolus, turn so as to face either one or the other source of light 

 and not show a resultant reaction by turning so as to face a point 

 midway between the two sources of light. It will be of much 

 interest to know what happens when such a test of Haberlandt's 

 ocellar theory is actually made. 



A Model for illustrating the Pilobolus Fruit-body in its Relations 

 with Light. — A model for illustrating the fruit-body of Pilobolus in 

 its relations with light can be made for demonstrations to an 

 audience as follows. Take a Florence flask or a measuring flask 

 with a somewhat pear-shaped base and of 500 cc. capacity, fill it 

 with water, stuff a smooth wet plug of cotton wool (free from air- 

 bubbles) down the neck as far as the base of the neck, and then close 

 the mouth of the neck with a cork (Fig. 60). The neck of the flask 

 then corresponds to the stipe of Pilobolus, the bulb of the flask to 

 the subsporangial swelling, and the cotton-wool plug to the proto- 

 plasmic septum at the top of the stipe. To represent the opaque 

 sporangium, stick a plano-convex mass of moulding clay covered 

 with black tissue paper over the flat surface of the flask's base, so as 

 just to cover it. As a source of light, use direct sunlight or a beam 

 from the arc of a projection lantern. 



1 Since this was written, Van der Wey, in a paper referred to at tlie end of 

 the last chapter (p. 43), has shown that, with small angles between the two beams 

 of light, both beams influence the direction in which the sporangiophores point, 

 with the result that most of the sporangia are shot in directions between the centres 

 of the two beams. Evidence of this fact is to be seen in one of his photographs 

 reproduced in my Fig. 16 (p. 44). 



2 G. Haberlandt, Die Lichtsinnesorgane der LaubhlitUer, Leipzig, 1905 ; also 

 Physiological Plant Anatomy, translated by M. Drummond, London, 1914, p. 614 

 et seq. 



